With the explosive growth of information services related to the “world wide web” in the last decade and the increasing demand for high resolution color graphic art and video in the material being presented therein, a demand for high data rate interconnects which are rapidly deployable, flexible and of minimum cost has been generated. With the high costs peripheral to the process of deploying fiber optics to satisfy this demand and the lack of flexibility in networks using fiber optics or other physically fixed installation, “Free Space Optical” (FSO) point to point data links have emerged which can fit these requirements. However, one significant drawback with FSO transceivers which has resulted in higher than expected cost and has forced units to be large and cumbersome is that once the transmit optical beams of the units have been aimed at the opposing receivers, motion in the platform supporting either unit or the influence of atmospheric air currents can be large enough to move either beam off the respective receiver aperture and thereby momentarily sever the data link. Another drawback is that to attain high-speed operation, the physical diameter of photosensitive receivers must be a fraction of a millimeter, thus leading to limitations on the Field of View (FOV) for the optical receiver systems. As with movement of the beam, the motion of the platform supporting the unit can be enough to move the image of the transmitter of the opposing unit outside the FOV of the receiver, again severing the optical link. This occurs even if the beam from the opposing unit remains on the receiver aperture.
A growing number of manufacturers have attempted to solve these difficulties by making the optical beams movable in order to track the motion of the opposite units. However, moving the beam along does nothing to resolve the problem of support motion moving the received beam outside the FOV of the receiver. A very few manufacturers have attempted to solve this latter problem by using a movable mirror to reflect both the outgoing beam to the opposing receiver and reflect the incoming beam into the receiver FOV. Herein lies a design problem related to the time scales of the motions introduced by a building, the support structure which attaches the unit to the building and the movement due to atmospheric currents which limits the ability to use the aforementioned beam steering approach. Overall movement of the building is driven by thermal expansion and the forces due to wind averaged over the building's mechanical resonance frequencies. As a result, these motions cause changes over many minutes or hours. The motion of the unit mount itself is due primarily to mechanical vibration in the building's structure proximal to the location of the mount and the forces on the unit due to wind averaged over the mechanical resonance frequencies of the mount. The resonance frequencies of the unit and its mount tend to be a small fraction of a Hertz up to a few Hertz, corresponding to a fraction of second to a small number of seconds; so the motions driven by wind tend to be of the same order in time. As most vibration sources, pumps and air handlers, are driven by alternating electrical current at 60 Hz, the motions from these sources tend to produce motions at fractions from 60 Hz to two or three times 60 Hz, corresponding to a fraction of a second in time down to many milliseconds. The influence of atmospheric air currents on an optical beam is fairly complicated and depends on momentary fluctuations in air temperature and speed but typically causes movement of the beam position on time scales similar to that outlined above for vibration.
The difficulty that these motion time scales produce is that the maximum speed of movement of the mirror, and hence the motions that can be compensated for, is limited by the mechanical resonance frequency of the mirror and its movement mechanism which is related to the physical size and mass of the mirror. A mirror fast enough to compensate for mount vibration and atmospheric air movement is typically a few millimeters diameter or less, which is much smaller than is practical for the minimum beam diameters attainable at typical link distances of a few hundred meters, and hence for practical receiver apertures. However, mirrors large enough to reflect the whole receiver FOV into the receiver aperture tend to be many centimeters diameter and to have mechanical resonance frequencies near or well below one Hertz. Thus, these mirror systems cannot compensate for mount vibration or atmospheric influences. Therefore, the receiver aperture must be made larger and the beam made larger to compensate for the further reduced speed of the tracking system. The result is that these systems can typically only compensate for overall building movement.
A further difficulty with implementing a motion tracking system is sensing the beam motion with sufficient precision at the opposing receiver in the presence of more intense light sources in the wavelength region of the laser transmitter, such as the sun. One implementation of a beam position sensor is to project part of the received beam onto a photosensitive detector with multiple segments. However, due to the relatively small size of these type of detectors relative to the typical receiver aperture diameter, a focusing lens or reducing telescope is needed to relay the sample of the received beam, which makes the sensing system fundamentally insensitive to the beam position. Instead, such a system senses the angle of arrival associated with the beam deflection which can be quite small, and the influence of atmospheric air currents can momentarily cause the beam to appear to emanate from a different angle than that joining the receiver and transmitter. An additional difficult with this system is that sunlight can enter the receiver aperture at an angle near the angle joining the units and fall unevenly on the segmented “position” sensor, thus causing the sensor signal to erroneously represent the beam source angle to be different from that of the transmitted beam. The size of the segments tend to be large enough that the response speed is much slower than the pulses representing the modulated transmit beam, so there is no possibility of distinguishing the signal due to light from the beam or from the sun, despite wavelength selective filtering.
These difficulties can be alleviated by using multiple individual sensors located above the periphery of the data receiver aperture for positioning sensing. The benefit of using individual sensor is that each sensor can be made with an FOV large enough that they are insensitive to the angle of arrival of the beam and therefore are able to sense the beam signal at distinct points in space. From this information, an approximate beam center position can be computed with potentially higher precision than that inferred from the angle of arrival. The individual sensors can be made small enough that they can respond to the pulses representing the modulated transit beam, thereby allowing electrical means to counter the sensitivity to sunlight or other sources in the wavelength region.
A search of the prior art did not disclose any patents that read directly on the claims of the instant invention, however the following U.S. patents are considered related:
U.S. Pat. No.INVENTORISSUED6,335,811Sakanaka1 Jan. 20025,867,294Sakai2 Feb. 19995,748,813Cassidy, et al5 May 1998
The 6,335,811 patent discloses an optical space communication apparatus. The apparatus includes a first electrical signal that is converted into a first optical signal, and the first optical signal is transmitted to a partner apparatus in the form of a first light beam. A second light beam transmitted from the partner apparatus is received to thereby detect a second optical signal by a photodetector of the apparatus. The second optical signal is converted into a second electrical signal. A transmission direction of the first light beam and the reception direction of the second light beam are changed in a direction to maximize the intensity of the second electrical signal obtained by converting the second optical signal detected by the photodetector.
The 5,867,294 patent discloses an optical space communication apparatus includes a transmission device for transmitting a first optical beam and a receiving device for receiving a second optical beam. A deflecting device is included for detecting the first and second optical beam.
The 5,748,813 patent discloses an optical communication system for free space communication. The system includes an optical source including a modulator to modulate the optical output to provide an optical signal. The optical source is connected to an optical fiber to pass the signal. The optical fiber includes an antenna having an optical fiber transmit portion with a core and a cladding. The cladding includes at least one substantially flat surface extending along its length and is arranged to couple light out of the optical fiber through the substantially flat surface(s).
For background purposes and as indicative of the art to which the invention is related reference may be made to the remaining cited patents.
U.S. Pat. No.INVENTORISSUED6,259,544Dishman, et al10 Jul. 20016,246,498Dishman, et al12 Jun. 20016,181,450Dishman, et al30 Jan. 2001